Calcium transients in neutrophils have been measured with the rapid mix flow cytometer. We have observed that the lag time for the response (2 seconds) is longer than the response time of the instrument (300 msecs). The ability to stimulate the cells uniformly has enabled us to detect cell populations in transition between minimal and maximal calcium levels. This project will be resumed with the installation of the new rapid mix device and Cytomation flow cytometer in year 18 to make possible simultaneous analysis of ligand-receptor interactions and cell response.